1. Field of the Invention
The present invention relates to a spread illuminating apparatus, and more particularly to a spread illuminating apparatus in which light exits out from both major surfaces of a light conductive plate.
2. Description of the Related Art
There has been a demand for a display device which has two viewing screens disposed in parallel with each other so as to sandwich an illuminating apparatus, and which is convenient for a situation where an electronics device, such as a notebook personal computer, is operated by a teacher, an instructor or a sales person while pupils, students or customers view images displayed. FIG. 10 is a schematic cross sectional view of a conventional display device for a notebook personal computer, introduced to answer the aforementioned demand. Such a display device is disclosed in, for example, Japanese Patent Application Laid Open No. H10-90678.
Referring to FIG. 10, a display device is foldably attached to a notebook personal computer. The display device comprises: a backlight system which comprises a cold-cathode fluorescent lamp 14 as a light source, a light conductive plate 11, and a reflector sheet 6 surrounding the cold-cathode fluorescent lamp 14; and two liquid crystal display (LCD) panels which each comprise a liquid crystal element 7, two glass plates 5 sandwiching the liquid crystal element 7, and two polarizer sheets 8 placed respectively on the two glass plates 5, and which are bonded respectively to both major surfaces of the light conductive plate 1 of the backlight system. Light emitted from the cold-cathode fluorescent lamp 14 is introduced into the light conductive plate 11 directly and indirectly via the reflector sheet 6 disposed surrounding the cold-cathode fluorescent lamp 14, and exits out entirely from the light conductive plate 11 through the both major surfaces thereby enabling images to be viewed at the both sides of the display device.
Under the circumstances, in recent years, more and more portable telephones are coming out with a built-in camera, and a person to be photographed, as well as a person to photograph, may want to monitor an image to be photographed from a side opposite to a side of the person to photograph. To meet this request, a display device comprises two LCD panels provided respectively at both sides of the portable telephone. In such a case, the two LCD panels may be different in display screen size, which raises a problem with the display device described above with reference to FIG. 12. Referring to FIG. 11, a person to photograph views one LCD panel (liquid crystal element 7A) from a direction P and a person to be photographed views the other LCD panel (liquid crystal element 7B) from a direction T. Here, the one LCD panel 7A has a larger display screen size than the LCD panel 7B. FIG. 11 also shows a light conductive plate 11, well-known polarizer plates 8, 8A and 8B, and a cold-cathode fluorescent lamp 14. Since the LCD panel 7A is larger than the LCD panel 7B, the light conductive plate 11 defines, on a side toward the LCD panel 7B, an area Y (hereinafter referred to as non-display area) which is not covered by the LCD panel 7B, and an area Z (hereinafter referred to as display area) which is covered by the LCD panel 7B. As shown in FIG. 11, light PR exiting out from the light conductive plate 11 toward the person to be photographed is not utilized at the non-display area Y and gone uselessly, which means wasted electric power of the light source thus having a critical impact on an electronics device, such as a portable telephone, which strongly demands low power consumption.
While light exiting out from the light conductive plate 11 at the non-display area Y is gone uselessly, light exiting out at the display area Z is incident on the LCD panel 7B and partly reflected thereby to be brought back into the light conductive plate 11, and then introduced in the LCD panel 7A to illuminate the LCD panel 7A. As a result, the person to photograph notices difference in brightness over the LCD panel 7A such that an area corresponding to the display area Z is brighter than an area corresponding to the non-display area Y.
In a 1.8 inch LCD used in a typical portable telephone, which is structured as shown in FIG. 11, for example, four light emitting diodes (LEDs) are disposed as light sources 1 as shown in FIG. 12B which, together with FIG. 12A, explains spot-by-spot brightness over the LCD panel 7A, where brightness of the LCD panel 7A measured at each spot indicated in FIG. 12A is shown in FIG. 12B (figures are relative values of actual measurement). Here, it is to be noted that the LCD panel 7B with a smaller display screen size is disposed away from the light sources 1 unlike shown in FIG. 11. FIG. 12B indicates that brightness is higher at spots located corresponding to the display area Z than at spots located corresponding to the non-display area Y, rendering a maximum difference of 1.4 times. That is, light emitted from the light sources 1 is wasted at the non-display area Y thereby causing uneven brightness over the LCD panel 7A.